I know this is probably simple enough, but the wording on Keeping Count leaves a little room for interpretation. Is the bonus equal to the difference between two copies of that card or is the bonus equal to all the tokens on the card that has more tokens?

It's the difference. One point of attack for [each token ... above the current number of ... tokens on this card]

I guess I'll just have to learn to use it differently then.

If you have two attackers, it can provide a moderate boost to one of them as they compete for the most kills. If you have only 1 dedicated attacker, you can put a second copy on someone else and set them up for one or two big attackers over the course of the game (+5 attack etc).

The card can be good in multiplayer if several players run copies in their decks. Then use shadow effects to discard lower copies and use retrieval to get it back without counters.

I gave it to another players Glorfindel with a Rivendell bow, so he could attack for 10+ later in the game after Haldir kept racking up the kills.